The thought of the battery charger: the grid wire is smaller than filament
- the grid wire causing problem will melt/fry/disconnect before the filament
does.
Your results may vary. This is posting of one day of test results, you
might not have same results (but could).
July 28, 2020 Midland, TX:
took the repaired 3-500 and inserted into AL82 for test
on energizing amp, as the filaments heated up, short returned..
De-energized, repeated high current injection with battery charger and fixed
again, this time no flash was seen inside tube.
Tube back into the AL82 ( I didn't write it down but think this time the
safety switch was open & no HV)
on energizing amp, as the filaments heated up, short returned..
Typically indicated by reverse reading on Grid current meter.
De-energized and pulled tube, but this time it tested OK with no shorts.
placed back into amp
on energizing amp, as the filaments heated up, short returned..
Pulled 3-500 tube and decided to make use of RF deck setup with 4-400 tubes
in other room
No HV issues as the HV power supply would not be on or even wired in at this
time.
removed one 4-400 tube and inserted the 3-500
connected battery charger battery cables between grid / filament (after
reviewing schematic to insure nothing would get fried)
but left battery charger line side unplugged.
Powered up filaments & Blower
applied line voltage to battery charger
slight quick additional glow seen in tube then gone, filaments still going.
De-energize
Tube tests good
into the AL82 it goes
Energize
no reverse grid current, 3400Vdc showing on HV meter, white hot filaments
glowing
wait few minutes while getting rig setup on 20 and lining up knobs on AL82
4 watt drive - all good, short does not return.
50 watts - all good
60 watts - all good - watch tubes, small red glow of plate seems equal
between tubes
70 watts - all good, similar - the plate glow is equal between tubes - this
setup has MFJ wattmeter showing 1350W out
Appears a Eimac 3-500Z can be saved. Your mileage (results) may vary.
If initial repair doesn't work, may have to put high current through
filament to grid short while filament is on.
I plan to use the AL82, of which one of the two tubes is the repaired Eimac
3-500 and give it a good RTTY work out, see if tube holds up OK.
CYA disclaimer: HV can kill, if you are not sure of that, don't remove
cover on amp..
73 W5AJ
Robert
Midland, Texas
From: Robert [mailto:woodr90@gmail.com]
Sent: Monday, July 27, 2020 7:47 PM
To: 'amps@contesting.com' <amps@contesting.com>
Subject: 3-500Z short / fix?
while repairing Zener diode - pulled two 3-500 Eimac tubes
one had short (all five pins read zero ohms to any other of the five pins)
took shorted tube to shop for a photo. sell tube as sample/light bulb.
Hummm
there is my old, really old car battery charger
tube is bad so why not
place battery charger in 6Vdc position
there are no protective shutdowns in this charger
(multi-tap transformer & one diode)
one clamp on two filament pins
other clamp on grid pin
let's see if grid will "melt" at area of the short
small glow/flash inside - then battery charger current drops to zero
go to 12vdc, no current
Measure ohms at tube pins:
filament good
Grids all connected
No short between filament/grid
shake tube - still OK
shake tube while measuring ohms, still OK
so, do put 3kV on the plate and see what happens??
anyone tried this before?
AL-82 test bed
W5AJ
Robert
Midland, Texas
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